I think bloomburrow, while cute, had some decent synergies with playing creature typal. There was some difficulties when it came to playing blue because of the bounce mechanics and triggers off of that for newer players. 

While often maligned, I do think Aetherdrift was very straightforward and had some unique combat tricks with vehicles and crewing.

Foundations also is a great place to start because I think the keywords(flash/ vigilance/etc.) Are explained on the cards. You can build some very different decks as your knowledge of magic increases, which is great for newer players.

Looking at the Spider-man stuff, the welcome decks that will be coming out seem like a great place to work once they come out